His letter to Gov. Deval Patrick and state lawmakers appears aimed at ensuring another Senate vote for healthcare reform if his cancer keeps him from being there. State Republicans are not impressed.

WASHINGTON — In one brief letter, Massachusetts Sen. Edward M. Kennedy underscored both the fragility of his own health and of the Democratic consensus on healthcare.

The ailing liberal leader wrote this week to his home-state governor, Deval Patrick, and state legislative leaders, asking them to alter the method by which Kennedy's successor could be chosen.

Massachusetts law calls for a special election to be held when a Senate vacancy arises, a process that would take months. Kennedy urged the Legislature to act to allow Patrick, a fellow Democrat, to appoint an interim replacement until such an election could be held.

Kennedy, who has brain cancer, wrote that he believes "it is vital for this commonwealth to have two voices speaking for the needs of its citizens and two votes in the Senate during the approximately five months between a vacancy and an election."

The health of Kennedy, 77, has been the subject of rampant speculation in Washington. He has rarely been seen this year in the Capitol, even as the Senate has taken up one of his lifelong causes, a revision of the nation's healthcare system.

He failed to appear on the Senate floor to cast a vote on the nomination of Supreme Court Justice Sonia Sotomayor.

White House Press Secretary Robert Gibbs, during his daily briefing, said that President Obama had last spoken to Kennedy in early June.

Kennedy's written request comes amid rising concerns in the Senate over the prospects of passing healthcare legislation this year. Democrats hold 58 seats in the chamber, and the body's two independents, Joe Lieberman of Connecticut and Bernie Sanders of Vermont, vote with the caucus. That would appear to provide a filibuster-proof majority.

But West Virginia's venerable Democratic Sen. Robert C. Byrd also is seriously ill and frequently misses votes.

Although Kennedy's letter never explicitly mentioned how a Senate vacancy in Massachusetts would occur and did not reference the healthcare debate, an interim appointee could be crucial in the event of the lawmaker's death or resignation.

The letter has sparked a small row in Massachusetts. Republicans in the state House were highly critical of his proposal, noting that in 2004 Democrats pushed through the measure that took away the governor's power to appoint an interim senator.

Massachusetts' governor at the time was Mitt Romney, and state Democrats were concerned that should Democratic Sen. John F. Kerry win the presidency, the Republican governor would choose his successor.

"This is just a political, partisan consideration," said Bradley Jones, the Massachusetts House minority leader.

He conceded that if Democrats, who hold a commanding grip on the state House, want to accede to Kennedy's wishes, they could change the law. "When the tanks get rolling, they're going to roll over us if they want to do that," Jones said.

Kennedy actually benefited from the former Massachusetts law. After his brother John won the presidency in 1960, a family friend was appointed to hold the seat until Edward Kennedy was old enough to run in a special election in 1962. He has served in the Senate ever since.